Sure, but just to throw the obvious example in to test this criteria, there have been times in history when people held other people as property, and other people engaged in violent action to curtail their right to own legitimate property. So what if extremists can be "right"? (at least temporarily)
You need to assess these things evaluating their goals, methods, outcomes (and whether the three are in alignment). If you're too quick to just label people as extremists then you yourself may be at risk of labelling those who disagree with you as axiomatically stupid, evil, mentally deficient or disabled or a combination of the above.